The present invention relates to linear direct-current (DC) permanent magnet motors and, more specifically, to linear DC permanent magnet servo motors and linear DC permanent magnet force motors.
Linear motion electric machines are relatively new, compared to rotating electric machines, but are finding increasing applications, because of the absence of gears or other rotary-to-linear converters. A review of linear motors may be found, for example, in the book "Linear Motion Electric Machines," published by Wiley, and authored by S. A. Nasar and I. Boldea. Prior development has concentrated primarily on synchronous or induction linear motors. Thus, a two-sided linear induction motor used for suspended vehicles is described in U.S. Pat. No. 3,895,585 to Peter Schwarzler. Linear, synchronous motors are disclosed in U.S. Pat. No. 3,706,922 and U.S. Pat. No. 3,594,622 to Junpel Inagaki. A propulsion arrangement employing a linear motor is disclosed in U.S. Pat. No. 3,884,154 to Fritz Marten. A linear stepper motor is disclosed in U.S. Pat. No. 4,037,122 to Gilbert Bonner and Joseph Dessel.
For some applications linear DC motors are preferable to linear induction or synchronous motors. For example, DC linear motors are particularly suitable for short-stroke applications. One of the factors limiting the use of linear electric motors is the low efficiency these motors have demonstrated in the past. This usually results from having only part of the windings under the poles at any one time, while power is being dissipated in all the windings. This is the limiting factor in the so-called voice coil motor. For this reason, voice coil motors with strokes longer than about one inch are very inefficient. Another limiting factor is the large attractive forces between various parts of the motors, which necessitate heavy supporting structures and/or close tolerance control of opposing elements having an air gap therebetween. Providing these forces and/or tolerances over long linear distances is expensive. Finally, until a very few years ago the peak forces produced by a linear motor were severely limited by demagnetization effects, except in the case of ceramic magnets, whose energy to weight ratio was too low for practical application in these motors.